Hydromechanical modeling of CO₂ injection into a saline aquifer: Analytical solution and fault-related seismicity assessment

The injection of substantial quantities of carbon dioxide into subsurface reservoirs may alter the stress state of geological formations, potentially reactivating pre-existing faults and triggering induced seismicity. Comprehensive hydromechanical coupling analytical approaches for predicting CO₂ injection-induced earthquakes remain underdeveloped. This study proposed an analytical solution for fully-coupled hydromechanical modeling of a saline aquifer due to CO₂ injection and applied it to the assessment of fault-related seismicity induced by CO₂ geological storage. Firstly, we derived the analytical solutions for pore pressure buildup and stress change, considering not only pore pressure diffusion but also poroelastic stressing and caprock stiffness. Then, we quantified the relative seismicity rates from Coulomb failure stress change using Dieterich's rate-and-state seismicity model. Subsequently, we investigated the occurrence rates and exceedance probabilities of CO₂ injection-induced earthquakes with different magnitudes according to a hybrid physical–statistical approach. Finally, we conducted a series of parametric studies to reveal the influence of several factors on induced seismicity. The results demonstrate several key findings. First, the proposed analytical solutions showed good agreement with multiphysics multiphase numerical simulation. Second, the traditional pure-hydraulic diffusion model overestimated the relative seismicity rate compared to the fully-coupled hydromechanical poroelastic model under a normal faulting stress regime. Third, the newly presented formula for the radius of the perturbed region was appropriate in reflecting the spatial-temporal evolution of seismicity rate. Finally, among the factors, the CO₂ injection rate had the largest impact on the occurrence rate and exceedance probability of induced earthquakes. In summary, this study established a comprehensive framework for evaluating CO₂ injection-induced seismicity according to fully-coupled hydromechanical analytical solutions.